In numerous publications on the subject of the preparation of propylene oxide, there are only a few which are concerned with integrated processes in which the energy of the vapor obtained in a distillation step is usefully returned to the process. This applies particularly to processes in which propylene oxide is separated off from solvents or traces of solvent by distillation.
WO-A-02/14298 describes a process for the continuous preparation of an olefin oxide. In the context of this process step, it is disclosed that the heat of condensation obtained at the top of a column can be recovered for one or all distillation processes of the overall process. In the column in question, a mixture comprising solvent, oxygen and inert gas is separated by distillation. Specific procedures for recirculating the heat of condensation are not disclosed.
WO-A-00/07965 describes a process for preparing propylene oxide, in which a mixture of propene, propylene oxide and methanol is separated off from a mixture via the top of a distillation column, with the reflux necessary for the separation in the column being condensed in a partial condenser at the top of the column.
If methanol, for example, is used as solvent in the preparation of propylene oxide from propene, it is generally advantageous for this to be used in the reaction section, i.e. for the reaction of propene with a hydroperoxide such as hydrogen peroxide, particularly when a titanium silicalite catalyst of the TS-1 type is used as catalyst for the reaction. On the other hand, the presence of methanol makes purification of the propylene oxide more difficult.
According to the prior art, at atmospheric pressure or superatmospheric pressures, essentially in the range from 1 to 5 bar, propylene oxide and methanol can be separated by distillation only when a distillation column having a very large number of theoretical plates is used and a very high reflux ratio is set at the same time, owing to the entraining azeotrope.
The separation task is simpler at lower pressures, but the low pressure has an adverse effect on the condensation temperature since the condensation temperature, which can, for example, be in the region of 15° C. depending on the pressure, requires provision of a high refrigeration power for condensation. Especially on an industrial scale, this incurs tremendous costs.
Other documents of the prior art relate to process where propylene oxide is separated from the solvent methanol by extractive distillation processes.
U.S. Pat. No. 5,849,938 discloses a process where propylene oxide is separated from methanol in a crude olefin epoxidation product by means of an extractive distillation wherein a relatively heavy polar solvent having hydroxy groups such as water or propylene glycol is used as the extracting solvent, propylene glycol being particularly preferred. According to this document of the prior art, the distillation column used ordinarily has from 20 to 60 theoretical plates, and the reflux/distillate ratio is generally in the range of from 5 to 15. According to the examples, a typical ratio is 9. Typical bottoms temperatures are in the range of 90 to 120° C., the pressure under which distillation is carried out being from 0.55 to 3.44 bar. According to the example, a preferred bottom pressure of the distillation column is 2.76 bar and therefore well above standard pressure. As typical propylene oxide fractions, fractions are obtained comprising 300 or 1,500 ppm of methanol. The bottoms streams obtained according to the examples comprise up to 6,300 ppm of propylene oxide. The purified propylene oxide stream obtained from the process according too U.S. Pat. No. 5,849,938 may be further purified and thus be subjected to a fractional distillation subsequently after removing the propylene oxide stream from the extractive distillation column.
U.S. Pat. No. 6,500,311 B1 discloses a process wherein a separation of methanol and propylene oxide takes place. As extracting solvent, a non-polar solvent, namely a C7-C9 hydrocarbon such as n-octane is used.
It is an object of the present invention to provide a method of separating propylene oxide from methanol which, compared to the processes described in the prior art, has an improved energy balance and, additionally, leads to top streams and bottoms streams having a lesser degree of impurity with regard to methanol and propylene oxide, respectively.
It is a further object of the present invention to provide a method of separating propylene oxide from methanol in which a cheap extracting solvent is employed which simultaneously allows for milder distillation conditions than those described in the prior art.
It is still another object of the present invention to provide a method which, compared to the processes described in the prior art, for example processes for separating propylene oxide from methanol or for preparing propylene oxide, has a significantly improved energy balance.
It is yet another object of the present invention to provide a method of producing propylene oxide in the course of which propylene oxide is separated from methanol wherein this separation has the above-mentioned advantages thus rendering the process for producing propylene oxide energetically and also with respect to the purity of the distillation fractions advantageous over the prior art.